The invention relates to a method for manipulating particles in fluidic microsystems, in particular for moving particles in microsystems along predetermined tracks, which are straight at least in sections, and devices for implementing such a method, in particular a fluidic microsystem in which synthetic or biological particles are manipulated in a suspension liquid, as well as applications of such a microsystem.
Fluidic microsystems with structures (e.g. channels) through which liquid flows, in which microelectrodes for influencing particles, (e.g. biological cells) through high-frequency fields on the basis of negative or positive dielectrophoresis are affixed to the channels through which liquid flows, have for example been described by G. Fuhr et al in xe2x80x9cNaturwissenschaftenxe2x80x9d, vol. 81, 1994, p. 528 ff.
Usually, a liquid flows through fluidic microsystems so as to move particles along. The microelectrodes attached to both the top and bottom of the longitudinal sides of the channel, result in compartmentalisation of the channel with high-frequency electrical fields by means of which the suspended particles can be deflected as desired, e.g. via branching out into adjacent channels or other structural elements. Above all the inflow of particles at one end of the channel and the setting of the flow speeds which as a rule are slow (some xcexcl/h) are associated with difficulties which with increasing miniaturisation impose increasingly severe limitations.
A general disadvantage of conventional fluidic microsystems is due to the fact that directional and adjustable particle movement requires a solution flow whose control (e.g. flow speed) causes considerable problems.
From the publication by M. J. Madou et al. in xe2x80x9cSPIExe2x80x9d, volume 3259, 1998, p. 80 ff, a centrifugal flow-through system is known in which flows of liquid in a microsystem are not regulated with conventional pumps and valves, but instead under the influence of centrifugal forces. To this effect the microsystem is in a disc-like carrier in the shape of a CD-ROM disc. Analogous to the operation of CD storage media, the carrier is intended to be spun at high speed (ranging from 100 to 10,000 revolutions per minute). The liquids in the microsystem move radially outward under the influence of the centrifugal forces. Simultaneously to this liquid movement, certain biochemical reactions take place in the microsystem. It is also intended that the movement of liquid be utilised for conveying particles, as is the case in a conventional pumped flow of liquid.
The centrifugal technology according to M. J. Madou et al is associated with the following disadvantages. Both the achievement of sufficient movement of fluid and the achievement of conveyance of particles, which as far as is possible is free from any obstructions, in the liquid in the disk-shaped flat rotor, necessitates the above-mentioned high revolutions of the carrier. This results in a limitation of the conventional centrifugal force flow-through system to particular basic functions of traditional centrifuging or to achieving biochemical reactions. The above-mentioned microelectrode technique for generating high-frequency electrical fields in the microstructures cannot be applied. There is a further disadvantage relating to particle sorting and particle counting achieved with conventional centrifugal techniques. Such sorting and counting is possible only if the size of microchannels created corresponds to the size of the particles to be processed. Therefore, any given microsystem is always restricted to a particular particle size. In addition, when handling biological particles (cells, cell components) interactions quickly occur between the particles and the channel wall, causing blockages of the channel.
Furthermore, centrifuge systems are generally known in which the sample material in the centrifuge is not only subjected to centrifugal forces but in addition also to magnetic or electrical forces so as to achieve specific separation effects depending on the relationship between centrifugal forces and additional forces. These centrifuge systems cannot however be used to manipulate biological objects. Biological objects (e.g. cells) are handled in relatively highly conductive solutions or suspensions (conductivity ranges from approx. 0.5 to 3 Siemens/m). In the case of conventional centrifuge systems with relatively large electrode surfaces, such conductivity would result in undesirable heating-up phenomena. Conventional centrifuge systems are therefore limited to a conductivity of approx. 0.1 Siemens/m.
It is the object of the invention to provide an improved method for manipulating particles in fluidic microsystems, which method overcomes the disadvantages of traditional microsystems and provides an extended application range. Furthermore it is the object of the invention to provide an improved fluidic microsystem with directional particle movement which can be adjusted simply and with high accuracy. It is also the object of the invention to provide applications for such an improved microsystem.
These objects are solved by methods and devices with the characteristics according to claims 1 or 10. Advantageous embodiments and applications of the invention are defined in the dependent claims.
A first important aspect of the invention consists of moving from the traditional centrifugal flow-through system with moved liquids, to a method where in a fluidic microsystem under the influence of centrifugal forces, only the particles to be manipulated are moved, with essentially no liquid flows or movements occurring in the microsystem. To this effect a number of measures are realised which in particular comprise the use of a fluidic microsystem closed off at least on one side, the provision of such a microsystem on an oscillatory rotor centrifuge, and operation of this centrifuge at a predetermined rotational speed at which the particles in the microsystem move as desired.
The method according to the invention allows centrifugal action at low speeds. Due to the use of an oscillatory rotor system where the rotor as the carrier of the microsystem is vertically aligned (at standstill or low speed), moving to a horizontal alignment (at high speeds), as the speed decreases, gravity increasingly influences the movement of the particles in the microsystem. According to a further aspect of the invention a particle movement in microsystems which are closed off on at least one side is also described, which when at a standstill is vertically aligned relative to the microsystem. Particle movement takes place as sedimentation under the influence of gravity.
According to the invention, in particular those types of microsystems which comprise microelectrode devices for dielectrophoretic influencing of particle movement, are combined with the principle of centrifuging. As a result of the centrifugal forces, the suspended particles move through the microchannels or other microstructures in a microsystem in which they (without being able to exit) are for example separated, brought to a predetermined position, fused, sorted or permeated under the influence of electrical polarisation forces.
The invention provides an important advantage in that for the first time in the case of microsystems with a complex structure, involving dielectrophoretic particle influencing, there is no need to use pumps or valves which are difficult to control and subject to malfunction, without this resulting in any limitation in the functionality of the microsystem. There are no limitations in relation to the dimensions of the channel cross sections. There is an option of rotating the microsystem simultaneously together with the associated control electronics. Interactions between particles (in particular biological particles) and wall areas of the microsystem can easily be prevented. Conversely, with respective structuring, such interactions can be achieved in a predetermined way for investigating binding procedures.
The invention provides an important advantage in that all particles can be subjected to the same extent to centrifugal forces, and can move corresponding to a reference direction along predetermined channels, and separation e.g. in various sub-channels or reservoirs is exclusively achieved via deflection forces which act in a particle-specific way, independent of the centrifugal forces. The direction of the deflection forces differs from the reference direction, with the angular difference being preferably less than 90xc2x0. Only the particle speed is set via the centrifugal force.
After separation, the additional forces can be switched off without the particles mixing again. It is an unexpected and important characteristic that through the use of a swinging rotor centrifuge, the contact between particles and sample chamber walls can be prevented. This is of importance especially in the case of biological objects.